A polymer electrolyte fuel cell is, for example, a stack of a plurality of cells each comprising a membrane/electrode assembly sandwiched between two separators. The membrane/electrode assembly comprises an anode and a cathode each having a catalyst layer and a polymer electrolyte membrane disposed between the anode and the cathode. For the polymer electrolyte membrane, a fluorinated proton conductive polymer such as a perfluorocarbon polymer having sulfonic groups is usually used. Further, the polymer electrolyte membrane is required to exhibit low electrical resistance.
In order to reduce the electrical resistance of a polymer electrolyte membrane, the polymer electrolyte membrane may be made thin. However, if the polymer electrolyte membrane is made thin, mechanical strength of the membrane decreases, whereby the membrane may be difficult to process or handle in production of a membrane/electrode assembly.
Further, the dimensions of the polymer electrolyte membrane are likely to increase in the lengthwise direction when the membrane contains water, thus leading to various drawbacks. For example, if the polymer electrolyte membrane is swelled by e.g. water formed by the reaction or water vapor supplied with a fuel gas and its dimensions increase, the electrode will follow the change of the dimensions of the polymer electrolyte membrane. However, since the membrane/electrode assembly is bound by e.g. separators, the increase in dimensions of the polymer electrolyte membrane leads to “wrinkles”. And grooves of the separators may be clogged with the wrinkles whereby the gas flow is impaired in some cases.
As a polymer electrolyte membrane having high mechanical strength even if it is thin and having excellent dimensional stability when it contains water, a polymer electrolyte membrane reinforced with a reinforcing material (e.g. porous body, fibrils, woven fabric or nonwoven fabric) has been proposed (Patent Documents 1 to 5).
In recent years, further decrease in the electrical resistance of a polymer electrolyte membrane has been desired. In order to further decrease the electrical resistance of a polymer electrolyte membrane, the concentration of ionic groups such as sulfonic groups contained in the fluorinated proton conductive polymer may be increased. However, if the concentration of ionic groups is significantly increased, the water content of the polymer electrolyte membrane per unit volume will significantly increase. This means that the change in the volume of the electrolyte membrane itself increases relative to the change in humidity of hydrogen gas or air assumed in practical operation of a fuel cell, and as a result, the durability will decrease. This tends to be significant as the operation temperature increases, and such is a fatal problem for the recent tendency of an increase in the operation temperature.
Accordingly, so long as a conventional fluorinated proton conductive polymer is used, there are limitations to a decrease in the thickness of the polymer electrolyte membrane and to an increase in the concentration of ionic groups contained in the fluorinated proton conductive polymer for the purpose of reducing the electrical resistance while mechanical strength and dimensional stability of the polymer electrolyte membrane are maintained.    Patent Document 1: JP-B-5-75835 (Claims)    Patent Document 2: JP-B-7-68377 (Claims)    Patent Document 3: JP-A-6-231779 (Claims)    Patent Document 4: WO04/011535 (Claims)    Patent Document 5: JP-A-2003-297394 (Claims, paragraphs 0012 and 0026)